A new study indicates soot, known as black carbon, plays a far greater role in global warming than previously believed and is second only to CO2 in the amount of heat it traps in the atmosphere. Reducing some forms of soot emissions — such as from diesel fuel and coal burning — could prove effective in slowing down the planet’s warming.

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It rises from the chimneys of mansions and from simple hut stoves. It rises from forest fires and the tail pipes of diesel-fueled trucks rolling down the highway, and from brick kilns and ocean liners and gas flares. Every day, from every occupied continent, a curtain of soot rises into the sky.

What soot does once it reaches the atmosphere has long been a hard question to answer. It’s not that scientists don't know anything about the physics and chemistry of atmospheric soot. Just the opposite: it does so many things that it's hard to know what they add up to.

To get a clear sense of soot — which is known to scientists as black carbon — an international team of 31 atmospheric scientists has worked for the past four years to analyze all the data they could. This week, they published a 232-page report in the Journal of Geophysical Research. "It's an important assessment of where we stand now," says Veerabhadran The new estimate of black carbon's heat-trapping power is twice that made by the IPCC. Ramanathan of the Scripps Institution for Oceanography, an expert on atmospheric chemistry who was not involved in the study.